1. Field of the Invention
The present invention relates to frequency tuning circuits and a voltage-controlled oscillator, and more particularly to a fine tuning circuit, a coarse tuning circuit and a voltage-controlled oscillator including the same that may operate efficiently and stably.
2. Description of the Related Art
Due to the widespread use of mobile communication, mobile communication terminals capable of offering better quality service are in demand. For instance, a voltage-controlled oscillator (VCO) is one of the devices that is used for mobile communication terminals, such as e.g., for a wide-band receiver. Also, the performance of the voltage-controlled oscillator significantly affects the quality of the mobile communication.
It is preferable that the voltage-controlled oscillator have a wide frequency tuning range and a low noise transmission. Such characteristics become more significant when the voltage-controlled oscillator is utilized at the front end of a mobile communication terminal, where noise may be transmitted through e.g., an antenna, or a digital cable.
A voltage-controlled oscillator includes an active circuit, an LC tank for frequency oscillation and a tuning circuit for tuning an oscillation frequency. The tuning circuit may be divided into a coarse tuning circuit and a fine tuning circuit.
Moreover, the tuning circuit may be implemented in a variety of forms. For example, a conventional tuning circuit typically includes a diode having a variable capacitance, wherein the diode has a structure of a p+/n− well junction. However, the above conventional tuning circuit may not achieve a higher Q value due to the use of the diode, and thus the performance of the tuning circuit may be lowered.
To overcome the above difficulty, an accumulation MOS (AMOS) varactor has been adopted for use in a tuning circuit.
FIG. 1 is a cross-sectional view illustrating a conventional structure of an AMOS varactor, which is also described in U.S. Pat. No. 6,211,745.
Referring to FIG. 1, the AMOS varactor may have a P-type gate formed on an N-type well (hereinafter, referred to as a P-Gate/N-well structure) or an N-type gate formed on a P-type well (hereinafter, referred to as an N-gate/P-well structure).
When the AMOS varactor has the P-gate/N-well structure, a source region 24 and a drain region 22 of the varactor are N+ types regions and well 20 of the varactor is an N− type well. Additionally, gate poly 30 of the P-type gate is of a P+ type conductivity and contacts 26 and 28 are composed of metal. A P-gate terminal 34 is connected to gate poly 30, and contact terminals 32 and 36 are connected to contacts 28 and 26, respectively. Moreover, when the AMOS varactor has the N-gate/P-well structure, the source region 24 and the drain region 22 are P+ types regions and the well 20 is a P− type well. Furthermore, gate poly 30 has an N+ type conductivity and contacts 26 and 28 are composed of metal.
FIG. 2 is a graph illustrating a tuning characteristic of the conventional AMOS varactor in FIG. 1. FIG. 2 shows that the capacitance Cmos of the AMOS varactor is varied according to a tuning voltage Vtune ranging from −2V to +2V,
As shown in FIG. 2, the capacitance Cmos of the AMOS varactor exhibits a significant change with respect to the tuning voltage Vtune ranging from about −1V to about 1V. In contrast, the capacitance Cmos of the AMOS varactor exhibits little change with respect to the tuning voltage Vtune outside of the range of about −1V to about 1V.
FIG. 3 is a circuit diagram illustrating a conventional fine tuning circuit. The conventional fine tuning circuit in FIG. 3 is also described in U.K. Patent Application No. GB2379104, Japan Patent Laid-Open Publication No. 2003-229718.
Referring to FIG. 3, the conventional fine tuning circuit includes a first varactor Cv1 and a second varactor Cv2 that are serially coupled to each other. The first varactor Cv1 and the second varactor Cv2 may include an AMOS varactor.
The first and second varactors Cv1 and Cv2 have gate terminals coupled to a first output terminal 11 and a second output terminal 12, respectively and source/drain terminals coupled to a tuning voltage input node 13 through which a tuning voltage Vtune is inputted. In addition, the first and second output terminals 11 and 12 correspond to an oscillation node of a voltage-controlled oscillation node.
The tuning voltage Vtune has a voltage level ranging from about 0V to a power supply voltage (VDD). For example, the power supply voltage may have a voltage level of about 2.8V. Moreover, the capacitance of the first varactor Cv1 and the second varactor Cv2 are varied according to the tuning voltage Vtune.
However, as shown in the graph of FIG. 2, the AMOS varactor exhibits a significant change in the capacitance with respect to the tuning voltage Vtune ranging from about −1V to about 1V. In contrast, the AMOS varactor exhibits little change in capacitance with respect to the tuning voltage Vtune outside of the range of about −1V to about 1V.
When a tuning voltage Vtune ranging from about 0V to about 2.8V is inputted, the capacitance tuning range of the AMOS varactor of less than 0V (for example, −1V) may not be produced. Consequently, the capacitance tuning range of the AMOS varactor may be reduced, by about one half of the full capacitance tuning range of the AMOS varactor, thereby rendering about half of the full capacitance tuning range of the AMOS varactor unavailable.
Therefore, in order to use the full capacitance tuning range of the AMOS varactor, the tuning voltage Vtune needs to range from a negative voltage level to a positive voltage level. However, in actual use, the tuning voltage Vtune may have a voltage range between ground and a power supply voltage (VDD).
In the conventional fine tuning circuit depicted in FIG. 3, a DC voltage outputted from the first and second output terminals 11 and 12 is provided to the first and second varactors Cv1 and Cv2 so that the capacitances of the first and second varactors Cv1 and Cv2 may vary according to the DV voltage outputted from the first and second output terminals 11 and 12.
Consequently, when a common noise is introduced to the DC voltage outputted from the first and second output terminal 11 and 12, the capacitances of the first and second varactors Cv1 and Cv2 may be modulated so that phase noise degradation may occur due to an FM modulation.
As described above, in the conventional fine tuning circuit, the capacitance tuning range of the AMOS varactor may be reduced. Moreover, the conventional fine tuning circuit may suffer degradation in performance due to, for example, phase noise degradation caused by noise introduced from the output terminal.
Additionally, the above difficulties may arise not only in the fine tuning circuit but also in a coarse tuning circuit, which has a circuit configuration very similar to the fine tuning circuit.
Thus, there is a need for a frequency fine tuning circuit, and/or a frequency coarse tuning circuit for use with a voltage-controlled oscillator, in which the capacitance tuning range of a varactor is increased while phase noise degradation due to an FM modulation is prevented.